Apparatus for testing a transformer to determine its ability to withstand voltage surges



1962 F. J. VOGEL 3,017,572

APPARATUS FOR TESTING A TRANSFORMER To DETERMINE ITS ABILITY To WITHSTAND VOLTAGE SURGES Filed Sept. 19, 1958 A W/W bo ww United States Patent APPARATUS l mit TESTING A TRANSFORMER T9 DETERMINE ITS AlilLlTY TO WITHSTAND VOLTAGE SURGES Fred J. Vogel, Pittsburgh, Pa, assignor to Allis-Chalmers Manufacturing Company, Milwaukee, Wis. Fiied Sept. 19, 1958, Ser. No. 764,211 Claims. (Cl. 324--55) This invention relates in general to transformers and in particular to a method of testing a transformer to determine its ability to withstand switching surge voltages without failure of its insulating structure. This application is a continuation in part of an application filed on July 13, 1956, Serial No. 597,798 now abandoned.

In designing insulation structures for transformers consideration must be given to various voltage conditions to which the transformer must be subjected during its service life. It is well known that most transformers will at various times during their life be subjected to fault voltages, lightning voltages and switching surges in addition to normal and sustained over voltages. The insulation design thus becomes complicated when consideration is given to these voltages having different magnitudes and different times of application. Transformers after being assembled and prior to being put into service are generally given a number of standard tests to determine their ability to withstand the different types of voltage conditions. These tests are designed to simulate actual voltage conditons which may be encountered by the transformer Without in any way weakening the insulation as a result of the test.

Standard tests now employed include a high potential test, an induced voltage test and an impulse test. The high potential test is made at full rated voltage, usually at sixty cycles, and is designed to determine the insulation between windings and from windings to ground is in good condition.

The induced voltage test is employed ,to simulate conditions which occur when the transformer is subjected to a fault voltage. The standard induced voltage test is usually made at twice normal voltage for a period of 7200 cycles. To keep the exciting current at a reasonable value over this current, the test is generally from 120 to 300 cycles for a sixty cycle transformer.

The standard impulse voltage test is employed to simulate voltage conditions which exist when the transformer or its lines are struck by lightning. The standard voltage wave for transformer impulse testing is called at 1 /z-4O wave. This voltage wave rises to its crest in approximately one and one-half microseconds and decreases to half the crest value in about less than forty microseconds. Standard tests on transformers consist of applying two voltage waves that are chopped after reaching their crest, followed by the 1 /2-40 wave. crest values at six or more times normal operating voltage.

Minimum values of voltages and characteristics of the voltage waves for each of the above tests have been established by the American Standards Association for each transformer voltage class. There is as yet no standard for the switching surge conditions that a transformer should be designed and tested to withstand. However,

the problems of switching surges on transformers have.

been of interest to numerous investigators who have produced a large body of information about the voltage conditions of actual switching surges, such as the article Switching Surges Due to Deenergization of Capacitive Circuits at page 562 of the August 1951 Power Apparatus and Systems.

A switching surge is an oscillation that is usually in the These waves may reachfrequency range of 120 to 1000 cycles per second. The actual frequency of a surge on a given transformer depends on various factors, such as the length of the lines being switched. A switching surge includes a crest oscillation that may be about twice the operating voltage and which is followed by a train of damped oscillations lasting for a few thousand microseconds. It would, of course, be advantageous to test a transformer for a some what more severe switching surge condition. It is well known that the desirable insulation level of a particular transformer depends in part on the transformer connec tions. Transformers with similar connections but in dilferent voltage classes generally have disproportionate insulation requirements. Although a switching surge test for a particular transformer should be selected with the individual requirements of the transformer in mind, a switching surge having a crest voltage on the order of three times the transformer operating voltage would in general be a satisfactory test. Thus a switching surge is somewhat lower in voltage and longer in duration than an impulse test, and it is somewhat higher in voltage and shorter in duration than the induced voltage test. The combined standard tests do not simulate a switching surge because the severity of a voltage condition depends on both the voltage and also on the duration of the condition.

In accordance with the present invention a very simple but effective test is provided by subjecting the transformer to a test wave that simulates the crest oscillation of a switching surge. If a transformer does not fail during the crest oscillation of an actual switching surge, it is not liable to fail on any of the smaller succeeding oscillations. The half cycle simulating the crest oscillation of a switching surge can very easily be applied to a transformer as a wave having a succession of cycles rapidly increasing in voltage to the desired crest. The small half cycles preceding the crest do not influence the test so long as the entire voltage wave is of a relatively short duration. The invention also provides a simple test device that uses the time constant of an alternating current generator field circuit to produce the desired test wave.

Therefore, it is an object of this invention to providea simple and inexpensive method for testing a transformer to determine its ability to withstand switching surge voltages.

Another object of this invention is to provide a test apparatus for simulating a switching surge in a transformer.

Other objects and advantages will begapparent from the following description when read in connection with the drawing in which:

FIG. 1 is a diagrammatic view of the circuit employed to test the transformer to determine its ability to withstand switching surge voltages;

FIG. 2 is a view of the voltage wave impressed on the terminals of the transformer by the circuit shown in FIG. 1; and

FIG. 3 is a view of half cycle of the wave shown in FIG. 2, drawn to a different time scale. I

As shown in FIG. 1, the test circuit comprises an alternating current generator 11, a suitable prime mover 12 for driving the generator, a generator field winding 13, a direct current generator 14 for exciting the field winding and a variable rheostat 15 and a switch 16 for controlling the excitation of the field winding. The test circuit also includes a means 19 for measuring and limiting the volt- I age wave applied to a test transformer 17. As illustrated the means 19 comprises a sphere gap connected to the terminals of the high voltage winding 20 of the trans former 17. The output of the alternating current generator 11 is connected to the low voltage winding 22 of the Patented Jan. 16, 1962 transformer 17, preferably through a current limiting impedance such as resistor 23.

After the transformer is connected as shown in FIG. 1, the generator 11 is driven by the prime mover 12 at a speed that corresponds to the desired frequency of the test wave. Preferably the frequency of the alternating current generator 11 is equal to the frequency of a switching surge that the transformer under test might encounter in service. A frequency of approximately 180 cycles per second is in general satisfactory for testing transformers. The switch 16 in the field circuit is kept open until the start of the test so that the field winding 13 is not excited and the generator 11 produces substantially no voltage. When the switch 16 is closed to start the test, the full Value of direct current from the generator 14 does not instantly flow through the field winding 13 but is limited by the resistance and the inductance of the field circuit. As is well known, the current applied to an inductive circuit increases with time according to the where R and L are the resistance and the inductance of the circuit and E is the applied voltage. As is also well known, the voltage output of a generator is in part a function of the current supplied to its field winding. Consequently, the voltage output of the alternating current generator 11 increases exponentially with successive half cycles as is indicated in the above equation and as is shown in FIG. 2.

Only a single half cycle of the test wave, as shown in FIG. 3, is necessary to simulate a switching surge; and it is desirable that the test wave reach the crest value rapidly. Preferably the test does not exceed thirty cycles and is on the order of one-tenth to two-tenths of a second. As is indicated by the above equation, the rate at which the voltage of the test wave increases is determined in part by the resistance in the circuit of the field winding 13. A simple means for changing the shape of the test wave is provided by the variable resistance 15 in the field circuit.

The sphere gap 19 measures the voltage of the test wave and removes the test Wave from the transformer when the crest voltage is reached. The voltage across the low voltage winding 22 depends in part on the impedance of the winding. The impedance of winding 22 is to a large extent determined by the impedance of the high voltage winding that is reflected into the low voltage winding 22. So long as the sphere gap 19 connected across the high y'oltage winding 20 is not conducting, the low voltage winding 22 has a high impedance and the voltage across winding 22 is approximately equal to the voltage output of generator 11. When the predetermined crest voltage is reached and the sphere gap 19 flashes over, the high voltage winding 20 is substantially short circuited and reflects only a low impedance into winding 22. As a consequence of the reduced impedance of the windings 20, 22, the voltage across both windings falls to a very low value.

Usually it will be desirable to connect the resistor 23 in series with the winding 22 to limit the current in the windings 20, 22 when the winding 20 is short circuited by the sphere gap 19.

In most instances a test voltage wave having a frequency of a. few hundred cycles per second and a crest value of three times normal voltage satisfactorily simulates switching surge voltages encountered by transformers. However, the test wave may be varied quite easily by changing the spacing of the sphere gap to either increase or decrease the crest value of the wave, by changing the speed of the prime mover to either increase or decrease the frequency of the wave, or by changing the setting of the resistance 15 to increase or decrease the rate at which the test wave builds up. It will thus be seen that a very simple and accurate switching surge voltage test is obtained by the present invention.

While only one embodiment of the present invention has been illustrated and described, it will be apparent to those skilled in the art that modifications other than those mentioned above may be made without departing from the spirit of the invention or from the scope of the appended claims.

What is claimed is:

1. Apparatus for testing a transformer for its switching surge strength comprising an alternating current generator having a voltage output that is a function of the current in a field producing winding, means for driving said generator at a speed corresponding to the frequency of an expected switching surge, means connected to said field Winding for supplying current according to the time constant of the field winding circuit to cause the voltage output of said generator to increase rapidly in an oscillatory manner, means for connecting the output of said generator to a winding of a transformer, and gap producing means connected to another winding of said transformer to limit the voltage at said transformer windings to a selected value.

2. Apparatus for testing a transformer for its ability to Withstand a switching surge having a predetermined frequency and a predetermined crest voltage, said apparatus comprising an alternating current generator connected across a winding of said transformer for plying a voltage wave having said predetermined frequency and having an oscillatory build-up to said predetermined crest in a relatively short time to simulate a switching surge, and means connected across a second winding of said transformer for short circuiting said second winding when said predetermined crest is reached to substantially remove said wave from said transformer.

3. Apparatus for testing a transformer for its ability to withstand a switching surge having a crest oscillation that is approximately three times the rated voltage of the transformer and having succeeding oscillations that are small relative to said crest, said apparatus comprising an alternating current generator connected across a first winding of said transformer for applying a voltage wave having a frequency of between and 1000 cycles per second and having a rapid oscillatory build-up to the voltage of said crest to simulate a switching surge while open circuiting the remaining windings of said transformer, means connected across one of said remaining windings for short circuiting said remaining winding when said test wave reaches said crest voltage to substantially remove said voltage wave from said windings and subsequently limit the current in said first winding.

4. Apparatus for testing a transformer for its ability to withstand a switching surge having a crest oscillation that is approximately three times the rated voltage of the transformer and having succeeding oscillations that are small relative to said crest, said apparatus comprising an alternating current generator means connected across a first winding of said transformer for applying a voltage wave having a frequency of approximately cycles per second and having an oscillatory build-up from substantially zero to the voltage of said crest within 30 cycles while open circuiting the remaining windings of said transformer, means connected across one of said remaining windings for short circuiting said remaining winding when said test wave reaches said voltage of said crest to substantially remove said voltage wave from said windings and subsequently limit the current in said first winding.

5. Apparatus for testing a transformer for its ability to withstand a switching surge comprising means connected across a high voltage winding of said transformer for adjusting the terminals of said high voltage winding to short circuit at a voltage of approximately three times the rated voltage of said high voltage winding, an alternating current generator connected across a low voltage winding of said transformer for applying a test wave having a frequency of approximately 180 cycles per second and having an oscillatory build-up Within 30 cycles to a voltage sufficient to short circuit said terminals and subsequently limit the current in said low voltage wind- References Cited in the file of this patent UNITED STATES PATENTS Dubsky Feb. 5, 1924 Wolfson Feb. 9, 1943 OTHER REFERENCES Rylander: Testing Insulation With High Frequency, Electronic Journal; January 1928; pages 10-43.

Dedication 3,017,572.F1"ed J. Vogel, Pittsburgh, Pa. APPARATUS FOR TESTING A TRANS- FORMER To DETERMINE ITs ABILITY TO WITHSTAND VOLTAGE SURGES. Patent dated J an. 16, 1962. Dedication filed Aug. 17, 1964, by the assignee, Allis-Chalmers Ma/rmfactm'ing Company. Hereby dedicates t0 the public the entire term of said patent.

[Ofiicial Gazette, 00t0b67" 6,1964] 

